Economical heavy concrete weight coating for submarine pipelines

ABSTRACT

Economical heavy concrete weight coating is used as a weight coating for submarine pipes. The developed economical heavy concrete weight includes steel slag and iron ore as aggregate. Steel slag can contain iron and thus have a higher density than some other types of slag. By supplementing the steel slag with iron ore as aggregate, the economical heavy concrete weight coating can have a higher density than a coating having just slag. Economical heavy concrete weight coating can have a density greater than 190 pcf.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation application of U.S. patentapplication Ser. No. 12/786,336, filed on May 24, 2010. For purposes ofUnited States patent practice, this application incorporates thecontents of the prior applications by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention generally relates to the field of submarine pipelines. Inparticular, the present invention is directed to an economical heavyconcrete weight coating used to keep the submarine pipeline submergedbelow the surface of the water.

2. Description of the Related Art

Concrete coating is applied on submerged offshore pipelines to provideadditional mass to ensure sufficient negative buoyancy. The minimumweight of the coating for this purpose is around 190 pounds per cubicfoot (“pcf”). Portland cement with natural aggregates is an inexpensivematerial that can weight concrete, but it does not meet the 190 pcfcriterion. Therefore, heavyweight aggregates, such as iron ore, are usedin lieu of natural aggregates. The proportion of iron ore aggregates maybe 75% of the total weight of the concrete coating. Consequently, thecost of the coating is high. It is desirable to have a concrete coatingthat achieves 190 pcf that is less expensive.

Various types of slag are produced during the production of iron andsteel. Iron is produced by combining scrap iron or iron ore with afluxing agent, such as limestone or dolomite, with a fuel, such as coke,in a blast furnace. The result is iron and blast furnace slag. The ironis frequently used as feedstock for a basic oxygen or electric arcfurnace to produce steel.

Blast furnace slag is a byproduct of iron production. It is generallynon-metallic and includes silicates, aluminosilicates, andcalcium-aluminino silicates. The blast furnace slag is often used inconcrete or asphalt. The most desirable, and thus common, use of blastfurnace slag is in the form of ground granulated blast furnace slag(“GGBFS”). The GGBFS is created by rapidly water-quenching the moltenslag to cool and solidify it. The rapid quenching produces a granulatedslag that includes sand-sized fragments with little or nocrystallization. The granulated slag can be ground (crushed or milled)to very fine cement-sized particles. Hence the name “ground granulatedblast furnace slag.” GGBFS has cement-like binding properties, whichmake it a suitable replacement for or additive to Portland cement.

Air-cooled blast furnace slag is blast furnace slag that is poured intobeds or molds and slowly cooled under ambient conditions. During theslow-cooling process, the blast furnace slag forms a crystallinestructure. The result is a hardened crystalline slag that can besubsequently crushed or molded. Pelletized blast furnace slag is avariation of air-cooled blast furnace slag. It is formed by cooling andsolidifying the blast furnace slag with water and then air quenching itin a spinning drum. The result is pelletized blast furnace slag having acrystalline structure. Air cooled blast furnace slag and pelletizedblast furnace slag are sometimes used as a lightweight aggregates inconcrete or asphalt. Aggregate is a hard, inert material (such as sand,gravel or slag) that is bound with a cementing material to formconcrete.

Steel is made by heating iron in a basic oxygen furnace or electric arcfurnace. Oxygen is injected into molten iron in the furnace. The oxygenreacts with impurities such as carbon monoxide, silicon, manganese, andphosphorus to form liquid oxides. The oxygen also reacts with some iron.The liquid oxides combine with lime and dolomite to form steel slag.This steel slag is different than blast furnace slag. Indeed, steel slagdoes not possess any cementing property as does GGBFS in crushed form.

Steel slag is disposed of as waste, or sometimes used in roadconstruction as aggregate in the granular base (the roadbed below thedriving surface), embankments, engineered fill, highway shoulders, andhot mix asphalt pavement.

SUMMARY OF THE INVENTION

Submarine pipes are used for a variety of applications, such as subseagas pipelines, subsea oil pipelines, and submarine pipes going toloading and unloading terminals for ocean-going tankers. The pipelinescan be weighted to ensure that they do not float to the surface where aship could strike them. Furthermore, weights are necessary to hold thepipes in place and thus prevent stress on the pipe and pipe joints.

Economical heavy concrete may used to weight subsea pipes. Theeconomical heavy concrete weight coating includes cement, iron ore,steel slag, and sand. Water can be used to activate the cement so thatit binds the other components together. Subsea pipes are coated with theeconomical heavy concrete weight coating before the pipes are placed inthe sea/ocean bed. The economical heavy concrete weight coating isattached to the pipe by, for example spraying, pouring, or compressingit onto the pipe.

Conventional concrete, which uses natural aggregate such as gravel andsand, does not have sufficient density to weight the subsea pipelines.Conventional concrete coating can use iron ore as an aggregate to weightthe pipe. The iron ore is relatively expensive and is not readilyavailable in some parts of the world.

Economical heavy concrete weight coating achieves the desired density byusing a combination of steel slag and iron ore as aggregate. Steel slagdoes not have cementious binding properties.

A device for weighting subsea pipes can include one or more pipesegments where each of the pipe segments has an exterior surface and afirst connector at a first end and a second connector at a second end,each of the first and second connectors positioned to connect each ofthe pipe segments to an adjacent one of the pipe segments. The devicecan also include an economical heavy concrete weight coating attached tothe exterior surface of the pipe segments, the economical heavy concreteweight coating having a unit weight greater than about 190 pounds percubic foot, and a compressive strength able to withstand 50 Newtons persquare millimeter. The economical heavy concrete weight coating caninclude a cementious compound that includes calcium compounds, silica,alumina, iron oxide, and gypsum; steel slag aggregate that includes30-35% iron oxide, 5-10% aluminum oxide, 20-25% calcium oxide, and 7-10%magnesium oxide, with a specific gravity greater than 3.2, and waterabsorption less than 3%; iron ore aggregate; and sand. The economicalheavy concrete weight coating can extend axially along the exteriorsurface of each pipe segment and have a radial thickness in a range of1-5 inches.

In one embodiment, the economical heavy concrete weight coating isattached to the exterior surface by first forming a ribbon of economicalheavy concrete weight coating and then compressing the ribbon ofeconomical heavy concrete weight coating onto the length of pipeline asthe length of pipeline is rotated about its axis. In one embodiment, theeconomical heavy concrete weight coating is attached to the length ofpipeline by pouring the economical heavy concrete weight coating into amold and having the length of pipeline located in the mold. In oneembodiment, the economical heavy concrete weight coating is attached tothe length of pipeline by spraying the economical heavy concrete weightcoating onto the length of pipeline.

In one embodiment, the iron ore comprises 40-45% of the total weight ofthe economical heavy concrete weight coating. In one embodiment, thecementious compound makes up no more than 17% of the total weight of theeconomical heavy concrete weight coating. In one embodiment, thecementious compound makes up at least 15% of the total weight of theeconomical heavy concrete weight coating. In one embodiment, theeconomical heavy concrete weight coating includes, by weight, 30-35%steel slag.

In one embodiment, a substantial portion of the steel slag aggregateincludes individual pieces of steel slag, wherein each of the individualpieces of steel slag have a width of 2.38 mm to 4.76 mm. In oneembodiment, the economical heavy concrete weight coating includes, byweight, 2-3% sand. In one embodiment, the economical heavy concreteweight coating includes, by weight, 5-6% water, at the time theeconomical heavy concrete weight coating is mixed.

In an embodiment of a technique for increasing the negative buoyancy ofpipe, the technique includes creating an economical heavy concreteweight coating that includes Portland cement; iron ore, the iron orebeing less than one half of the weight of the economical heavy concreteweight coating; steel slag, the steel slag produced by purifying steelbillets in an electric arc furnace or a basic oxygen furnace; and sandto give the economical heavy concrete weight coating a unit weight of atleast about 190 pounds per cubic foot and a thickness in a range of 1-5inches. The technique also includes the steps of attaching theeconomical heavy concrete weight coating to an outer surface of one ormore segments of pipe; connecting each of the segments of pipe to anadjacent segment of pipe to create a length of subsea pipeline;submerging the length of subsea pipeline in a body of water. The subseapipeline has negative buoyancy that maintains the length of subseapipeline below a surface of the body of water.

In one embodiment, the steel slag makes up 40-45 percent of theeconomical heavy concrete weight coating. In one embodiment, the step ofattaching the economical heavy concrete weight coating to the outersurface of each of the segments of pipe includes compressing theeconomical heavy concrete weight coating onto the outer surface. In oneembodiment, the step of attaching the economical heavy concrete weightcoating to the outer surface of each of the segments of pipe includesspraying the economical heavy concrete weight coating onto the outersurface.

In one embodiment of an apparatus for weighting subsea pipelines, theapparatus includes an economical heavy concrete weight coating thatincludes 40 to 45% iron ore; 16-17% cement; 2-3% sand; and 30 to 35%steel slag. The steel slag includes 30-35% iron oxide; 5-10% aluminumoxide; 20-25% calcium oxide; 7-10% magnesium oxide. The apparatus alsoincludes a segment of pipeline, wherein the economical heavy concreteweight coating covers a substantial portion of an exterior surface ofthe segment of pipeline.

In one embodiment, the steel slag does not have cementious bindingproperties. In one embodiment, the radial thickness of the economicalheavy concrete weight coating is less then five inches. In oneembodiment, the unit weight of the economical heavy concrete weightcoating is at least 190 pcf. In one embodiment, the unit weight of theeconomical heavy concrete weight coating is less than 210 pcf.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an orthogonal view of an exemplary embodiment of economicalheavy concrete weight coating attached to a subsea pipe.

FIG. 2 is a perspective view of an exemplary embodiment of a techniqueto attach the economical heavy concrete weight coating to the subseapipe of FIG. 1.

FIG. 3 is a perspective view of another exemplary embodiment of atechnique to attach the economical heavy concrete weight coating to thesubsea pipe of FIG. 1.

FIG. 4 is a side view of segments of the subsea pipe of FIG. 1positioned for lowering through a body of water.

FIG. 5 is a side view of segments of the subsea pipe of FIG. 1positioned on a sea floor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawing which illustrates embodiments ofthe invention. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theillustrated embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout, and the prime notation,if used, indicates similar elements in alternative embodiments.

Referring to FIG. 1, submerged offshore pipelines can have naturalbuoyancy that either causes the pipe to float towards the surface orcreates stress on pipe joints and connections. The buoyancy problem canbe controlled by applying weights to the pipe to ensure the pipe hassufficient negative buoyancy. One type of weight is an economical heavyconcrete weight coating. A heavyweight pipe assembly 100 is created byapplying an economical heavy concrete weight coating 102 to a length ofsubsea pipe 104. Subsea pipe 104, also referred to as submarine pipe,can be any type of pipe intended to be used while submerged in a body ofwater. The type of water includes sea water or fresh water. Theeconomical heavy concrete weight coating 102 adheres or is otherwiseattached to an exterior surface 106 of the length of pipe 104.Preferably, the economical heavy concrete weight coating 102 extendscircumferentially around exterior surface 106 and extends axially tocover a substantial length of pipe segment 104. Connectors 108, whichcan be threads or exposed surfaces suitable for welding (not shown) canbe located at the ends of pipe segment 104. Preferably, the economicalheavy concrete weight coating 102 does not cover connectors 108. Thehigh density of the economical heavy concrete weight can provide enoughnegative buoyancy to reduce or eliminate the pipeline's tendency tofloat towards the surface. The desirable unit weight of a suitablecoating is around 190 pounds per cubic foot (“pcf”). A coating with alower unit weight may not provide adequate negative buoyancy, or mayrequire a large volume of concrete, thus making the pipe assemblyunwieldy.

In one embodiment, economical heavy concrete weight includes cement andaggregates. The cement can be any cementious compound that hassufficient binding properties for the condition in which it will beused. For example, the cement is capable of binding the economical heavyconcrete weight with enough strength to resist failure during placement.The cement can also bind the economical heavy concrete weight to thelength of pipe 104. Furthermore, in some embodiments, the cementmaintains its binding properties while exposed to sea water or freshwater.

Cement can be, for example, any type of portland cement, as defined byASTM Specification C-150. In one embodiment, the cement includes calciumcompounds, silica, alumina, iron oxide, and gypsum. The cement can be atype that resists sulfate attack, such as, for example, Type V portlandcement (as defined by ASTM C150) or any hydraulic cement meeting therequirements of Type HS (as defined by ASTM C1157). In one embodiment,pozzolan, or a blend of pozzolan and portland cement is used as acementious compound. Pozzolan, when added to portland cement, canincrease the strength of the cement and can also increase resistance tosulfate attacks.

The cementious compound can be activated with water and used to bindaggregates and, thus, form economical heavy concrete weight. Typicalconcrete made with Portland cement and natural aggregates may not meetthe 190 pcf criterion for concrete coating suitable to weight pipesegments. Therefore, high density aggregates are added to the concreteto increase the weight of the concrete. One high density aggregate canbe iron ore. However, if iron ore is the only high density aggregate,concrete coating may include as much as 75%, by weight, iron ore inorder to achieve 190 pcf.

In one embodiment of economical heavy concrete weight coating, theaggregates can include iron ore, sand, and steel slag. In a preferredembodiment of economical heavy concrete weight coating, both steel slagand iron ore are used as aggregates. Other aggregates may also be used.In some embodiments, the economical heavy concrete weight coatingincludes 40-45% iron ore, 30-35% steel slag, 16-17% cement, 2-3% sand,and 5-6% water at the time it is mixed. The water can be consumed in achemical reaction when the cement cures.

Steel slag is the slag produced when steel billets are purified in anelectric arc furnace or a basic oxygen furnace. Unlike blast furnaceslag, steel slag does not have cementious binding properties. Steel slagaggregates can have a generally spherical shape or an irregular shape.Typical steel slag aggregates may range between approximately 2.38 mmand approximately 4.76 mm in diameter or in cross-sectional width. Inone embodiment economical heavy concrete weight coating, a substantialportion of the steel slag aggregate includes individual pieces of steelslag, wherein each of the individual pieces have a width of about 2.38mm to 4.76 mm. Steel slag has a higher density than natural aggregateand, thus, can be used to create concrete having a higher density thanconcrete having natural aggregate. Furthermore, steel slag can containiron, giving it a greater density than blast furnace slag. Preferably,steel slag used in ECH has a bulk specific gravity of at least about3.4. In one embodiment, steel slag for use as economical heavy concreteweight coating can include by weight, about 10-40% iron oxide andpreferably 30-35% iron oxide, 5-10% aluminum oxide, 20-25% calciumoxide, and 7-10% magnesium oxide. In another embodiment, steel slag caninclude about 10-40% iron oxide, 40-52% calcium oxide, and 10-19%silicon oxide. Steel slag is an industrial waste product that is adisposed problem. Economical heavy concrete weight is an environmentallyfriendly, beneficial use of an industrial waste product that mayotherwise end up in a land fill.

Steel slag aggregates, without iron ore, can be used to increase thedensity of concrete, but the density may be insufficient to function aseconomical heavy concrete weight coating. As shown in the followingtable, an embodiment having steel slag as the only high-densityaggregate results in a unit weight of 176 pcf, which is less than thedesired unit weight of 190 pcf.

Material Weight % Average Unit Weight (pcf) Cement 14.06 176 Iron ore —Slag aggregate 79.7 Sand 1.4 Water* 4.78 *Value reflects amount of waterpresent at the time the concrete coating is mixed.

Due to the lower unit weight associated with concrete coating whereinsteel slag is the only high density aggregate, a relatively thickcoating is used to provide an amount of negative buoyancy to a length ofsubmarine pipe operable to maintain submersion. Indeed, a concretecoating having only steel slag as an aggregate is significantly thickerthan a concrete coating made with iron ore aggregate.

As described, above, a binding agent, such as cement, is used to bindthe iron ore, steel slag, sand, and any other components of theeconomical heavy concrete weight coating. An unexpected benefit,however, is that, in spite of the lack of cementious binding propertiesin steel slag, the quantity of cement operable to bind economicalconcrete is less than the quantity of cement operable to bind Portlandcement or concrete coating having just iron ore as aggregate. In someembodiments, the quantity of cement used in economical heavy concreteweight coating can be about 15-16%, by weight, rather than the 17.5%cement used for concrete coating having iron-ore aggregate and anabsence of steel slag. Another unexpected result of economical heavyconcrete weight coating having both steel slag and iron as aggregate isa compressive strength of more than about 50 N/mm².

Given the same volume, economical heavy concrete weight can be 30 to 40%heavier than standard Portland cement concrete. Indeed, economical heavyconcrete weight coating having both steel slag and iron ore as aggregatecan have a unit weight greater than about 190 pcf. In an exemplaryembodiment, the economical heavy concrete weight coating has a unitweight of about 200 pcf. Part of the density of economical heavyconcrete weight is due to the iron content in the steel slag. Indeed,the steel slag has a higher density than blast furnace slag and thus canbe used to create higher density concrete. Furthermore, due to theincreased iron ore aggregate content, economical heavy concrete weightcoating containing both steel slag and iron ore as aggregate achieveshigher unit weight than a coating having just steel slag as anaggregate. The economical heavy concrete weight coating can, however,have a unit weight below 210 pcf, as shown in the following tables. Thefollowing table shows composition, by weight, of an exemplary embodimentof economical heavy concrete weight coating having steel slag and ironore as aggregate.

Material Weight % Average Unit Weight (pcf) Cement 15.93 207 Iron ore55.02 Slag aggregate 19.66 Sand 3.94 Water* 5.42 *Value reflects amountof water present at the time the economical heavy concrete weightcoating is mixed.

The following tables illustrate the unit weight of concrete coatinghaving heavy aggregate of iron ore and economical heavy concrete weightcoating 102 having aggregates of iron ore and steel slag:

TABLE 1 Unit weight development of concrete coating having only ironore. Unit Avg. Sample Age, Length, Width Depth Weight, Wt., Unit # Dayscm cm cm gr Pcf Wt. Pcf 1 7 10.11 10.16 10.138 3671.3 219.8 220.77 210.02 10.10 10.188 3653.9 220.94 3 10.22 10.22 10.09 3745.3 221.56 4 1410.018 10.20 10.04 3642.7 221.37 222.08 5 10.08 10.10 10.1 3655 221.61 610.00 10.192 10.11 3690 223.26 7 28 10.038 10.26 10.086 3653.8 219.30220.43 8 10.20 10.00 10.16 3669.3 220.79 9 10.11 10.01 10.266 3686.4221.22

TABLE 2 Unit weight development of concrete coating having only ironore. Unit Avg. Sample Age, Length, Width Depth Weight, Wt., Unit # Dayscm cm cm gr Pcf Wt. Pcf 1 7 10.07 10.10 10.21 3143.5 188.73 190.93 210.12 10.08 9.97 3134.7 192.16 3 10.14 10.23 10.12 3231.5 191.92 4 1410.09 10.09 10.13 3133.4 189.42 189.24 5 10.12 10.00 10.15 3129.3 189.936 10.07 10.15 10.10 3120.3 188.37 7 28 10.08 10.03 10.136 3151.3 191.72191.10 8 10.19 9.97 10.17 3172.3 191.46 9 10.00 10.078 10.18 3128.7190.13

TABLE 3 Unit weight development of economical heavy concrete weightcoating having iron ore and steel slag aggregate. Unit Avg. Sample Age,Length, Width Depth Weight, Wt., Unit # Days cm cm cm gr Pcf Wt. Pcf 1 710.17 10.04 10.07 3354 203.37 203.87 2 10.03 10.07 10.142 3360 204.58 310.04 10.186 10.05 3356 203.65 4 14 10.17 10.16 10.24 3430 202.11 202.455 10.10 10.17 10.05 3362 203.04 6 10.04 10.158 10.14 3354 202.20 7 2810.024 10.214 10.264 3414.3 202.56 203.74 8 10.00 10.1 10.138 3364.5204.86 9 10.14 10.128 10.06 3376.1 203.81

TABLE 4 Unit weight development of economical heavy concrete weightcoating having iron ore and steel slag aggregate. Unit Avg. Sample Age,Length, Width Depth Weight, Wt., Unit # Days cm cm cm gr Pcf Wt. Pcf 1 710.138 10.198 10.12 3446.2 205.35 204.06 2 10.27 10.2 9.97 3418.7 204.003 10.24 10.12 10.096 3403.6 202.82 4 14 10.084 10.18 10.17 3395.1 202.75202.92 5 10.13 10.066 10.25 3397.8 202.64 6 10.20 10.05 10.2 3410.7203.37 7 28 10.228 10.07 10.084 3409.4 204.66 204.68 8 10.25 10.17 10.033428 204.41 9 10.16 10.02 10.16 3399.1 204.97

Subsea pipe 104 is available in a variety of sizes. Economical heavyconcrete weight coating can be used as a coating on any size subseapipe. The outer diameter of the subsea pipe 104 may range from 16-48inches, or could be larger or smaller. The thickness of the coating mayrange from about one inch to five inches thick, thus making the outerdiameter of the economical heavy concrete weight coating about 17 to 53inches.

The economical heavy concrete weight coating 102 can be attached topipes 104 or other subsea equipment (not shown) by various applicationtechniques, including spraying, pouring, or compressing it into place.The economical heavy concrete weight coating can be attached by adheringto the pipe segment, by mechanical fasteners such as bolts, clamps, orrings, or by other suitable techniques. Preferably, connectors such asthreads on the ends of pipe segment 104 are not covered with economicalheavy concrete weight coating.

The spraying technique of attaching economical heavy concrete weightcoating 102, shown in FIG. 2, involves mixing the cement 110, iron ore112, steel slag 114, sand 116, and water 118 in a mixing container, suchas hopper 120. The economical heavy concrete weight coating 102 can thenbe sprayed by nozzle 122 onto pipe segment 104 to cover a substantialportion of the exterior surface of the pipe segment. In one embodiment,pipe segment 104 can be rotated and moved axially below a fixed nozzle122. The cementious properties of the economical heavy concrete weightcoating 102 can cause the economical heavy concrete weight 102 to adhereto the pipe segment 104. In another embodiment, pipe segment 104 isrotated while nozzle 122 moves axially. In still another embodiment,pipe segment 104 can remain stationary while nozzle 122 moves axiallyand circumferentially about pipe segment 104.

As shown in FIG. 3, in another embodiment, the economical heavy concreteweight coating 102 is attached by being compressed onto pipe segment104. In this embodiment, cement 110, iron ore 112, steel slag 114, sand116, and water 118 are mixed in a mixing container such as hopper 124. Astill-wet, and thus pliable, economical heavy concrete weight ribbon 126comes out of nozzle 128 and is compressed onto pipe segment 104. Pipesegment 104 is rotated about its axis as the ribbon 126 is axiallyadvanced along its length. A conveyor 130 can be used to compress ribbon126 into place on pipe segment 104. Thus, the economical heavy concreteweight coating 102 adheres to the pipe segment 104 as it dries. Thepouring technique (not shown) involves building a formwork around thepipe 104 and then pouring the economical heavy concrete weight into theformwork.

The pipe segments 104 having economical heavy concrete weight coating102 can be used in a variety of ways. For example, they can be used as asubsea pipeline. In one embodiment, they are joined together and placedon the floor of the sea. One technique to place the pipe segments 104 onsea floor 140 involves joining the connectors 108 of adjacent pipesegments 104 to form a length of pipeline. The pipe segments 104 arethen suspended by cables 146 from buoyant hangers 142 on the surface 144of a body of water, as shown in FIG. 4. The cables 146 of a portion ofthe buoyant hangers 142 can then be lowered at the approximately thesame time and rate so that the length of pipeline remains relativelystraight as it is lowered toward the sea floor 140. This minimizesstrain on connectors 108 and also minimizes flexing of each pipe segment104. Minimal bending can reduce cracking on the economical heavyconcrete weight coating 102.

As shown in FIG. 5, the pipeline made of pipe segments 104, each havingeconomical heavy concrete weight coating 102, rests on the sea floor140. The economical heavy concrete weight 102, having a weight of, forexample, 200 pounds per cubic foot, prevents the pipe segments 104 fromrising off of the sea floor 140. In one embodiment, wherein lessnegative buoyancy is acceptable, a portion of the pipe segments 104 canbe free of economical heavy concrete weight coating 102. For example,every fourth pipe segment 104 can be coating free, while the other threesegments have economical heavy concrete weight coating 102. In anotherembodiment, where less negative buoyancy is required, the coating can beless thick. For example, and embodiment requiring less negative buoyancymay have a one inch thick economical heavy concrete weight coating 102,whereas an embodiment requiring more negative buoyancy may have a fiveinch thick coating.

What is claimed is:
 1. A method for increasing the negative buoyancy ofa segment of pipe, the method comprises: creating an economical heavyconcrete weight coating comprising: Portland cement; iron ore, where theiron ore is less than one half of the weight of the economical heavyconcrete weight coating; steel slag, where the steel slag is produced bypurifying steel billets in one of an electric arc furnace or a basicoxygen furnace; and sand; where the economical heavy concrete weightcoating has a unit weight of at least about 190 pounds per cubic foot(pcf) and a thickness in a range of about 1 to about 5 inches; andattaching the economical heavy concrete weight coating to an outersurface of the segment of pipe, where the segment of pipe with theeconomical heavy concrete weight coating has a greater negative buoyancythan without the economical heavy concrete weight coating.
 2. The methodaccording to claim 1 where the steel slag comprises about 40 weightpercent (wt. %) to about 45 wt. % of the economical heavy concreteweight coating.
 3. The method according to claim 1 where attaching theeconomical heavy concrete weight coating to the outer surface of thesegment of pipe comprises compressing the economical heavy concreteweight coating onto the outer surface.
 4. The method according to claim3 where attaching the economical heavy concrete weight coating to theouter surface of the segment of pipe also comprises forming a ribbon ofeconomical heavy concrete weight coating and rotating the segment ofpipe about its axis.
 5. The method according to claim 1 where attachingthe economical heavy concrete weight coating to the outer surface of thesegment of pipe comprises spraying the economical heavy concrete weightcoating onto the outer surface.
 6. The method according to claim 5 whereattaching the economical heavy concrete weight coating to the outersurface of the segment of pipe further comprises rotating the segment ofpipe and spraying the economical heavy concrete weight coating using anozzle that moves axially along the segment of pipe.
 7. The methodaccording to claim 5 where attaching the economical heavy concreteweight coating to the outer surface of the segment of pipe furthercomprises spraying the economical heavy concrete weight coating using anozzle that moves both axially and circumferentially about the segmentof pipe while the segment of pipe remains stationary.
 8. The methodaccording to claim 1 where attaching the economical heavy concreteweight coating to the outer surface of the segment of pipe comprisespouring the economical heavy concrete weight coating into a mold, wherethe outer surface of the segment of pipe is located within the mold. 9.The method according to claim 1 where the economical heavy concreteweight coating has a unit weight of at least about 210 pcf.
 10. Themethod according to claim 1 where the economical heavy concrete weightcoating comprises about 40 wt. % to about 45 wt. % iron ore; about 16wt. % to about 17 wt. % Portland cement; about 2 wt. % to about 3 wt. %sand; and about 30 wt. % to about 35 wt. % steel slag.
 11. The methodaccording to claim 10 where the steel slag comprises about 30 wt. % toabout 35 wt. % iron oxide; about 5 wt. % to about 10 wt. % aluminumoxide; about 20 wt. % to about 25 wt. % calcium oxide; and about 7 wt. %to about 10 wt. % magnesium oxide.
 12. A method for providing negativebuoyancy to a length of subsea pipeline, the method comprising: creatingan economical heavy concrete weight coating comprising: Portland cement;iron ore, where the iron ore is less than one half of the weight of theeconomical heavy concrete weight coating; steel slag, where the steelslag is produced by purifying steel billets in one of an electric arcfurnace or a basic oxygen furnace; and sand; where the economical heavyconcrete weight coating has a unit weight of at least about 190 poundsper cubic foot and a thickness in a range of about 1 to about 5 inches;attaching the economical heavy concrete weight coating to an outersurface of each of a plurality of segments of pipe; connecting each ofthe plurality of segments of pipe to an adjacent one of the plurality ofsegments of pipe to create the length of subsea pipeline; and submergingthe length of subsea pipeline in a body of water, where the length ofsubsea pipeline has a negative buoyancy and is operable to maintain thelength of subsea pipeline in a position below a surface of the body ofwater.
 13. The method according to claim 12 where attaching theeconomical heavy concrete weight coating to the outer surface of each ofthe plurality of segments of pipe forming the length of subsea pipelinecomprises compressing the economical heavy concrete weight coating ontothe outer surface of each of the plurality of segments of pipe formingthe length of subsea pipeline.
 14. The method according to claim 13where attaching the economical heavy concrete weight coating to theouter surface of each of the plurality of segments of pipe forming thelength of subsea pipeline comprises forming a ribbon of economical heavyconcrete weight coating and rotating the length of subsea pipeline aboutits axis.
 15. The method according to claim 12 where attaching theeconomical heavy concrete weight coating to the outer surface of each ofthe plurality of segments of pipe forming the length of subsea pipelinecomprises spraying the economical heavy concrete weight coating onto theouter surface of each of the plurality of segments of pipe forming thelength of subsea pipeline.
 16. The method according to claim 15 whereattaching the economical heavy concrete weight coating to the outersurface of each of the plurality of segments of pipe forming the lengthof subsea pipeline further comprises rotating the length of subseapipeline and spraying the economical heavy concrete weight coating usinga nozzle that moves axially along the length of subsea pipeline.
 17. Themethod according to claim 15 where attaching the economical heavyconcrete weight coating to the outer surface of each of the plurality ofsegments of pipe forming the length of subsea pipeline further comprisesspraying the economical heavy concrete weight coating using a nozzlethat moves both axially and circumferentially about the length of subseapipeline while the length of subsea pipeline remains stationary.
 18. Themethod according to claim 12 where attaching the economical heavyconcrete weight coating to the outer surface of each of the plurality ofsegments of pipe forming the length of subsea pipeline comprises pouringthe economical heavy concrete weight coating into a mold, where theouter surface of the length of subsea pipeline is located within themold.
 19. The method according to claim 12 where the economical heavyconcrete weight coating comprises about 40 wt. % to about 45 wt. % ironore; about 16 wt. % to about 17 wt. % Portland cement; about 2 wt. % toabout 3 wt. % sand; and about 30 wt. % to about 35 wt. % steel slag. 20.The method according to claim 19 where the steel slag comprises about 30wt. % to about 35 wt. % iron oxide; about 5 wt. % to about 10 wt. %aluminum oxide; about 20 wt. % to about 25 wt. % calcium oxide; andabout 7 wt. % to about 10 wt. % magnesium oxide.